Evaporative water cooling towers are well known in the art. These towers have been used for many years to reject heat to the atmosphere. Evaporative water cooling towers may be of many different types including counterflow forced draft, counterflow induced draft, crossflow forced draft, crossflow induced draft, hyperbolic, among others.
Evaporative water cooling towers are used in a variety of applications. For example, such towers are used to provide cooling water to industrial processes such as food processing operations, paper mills, and chemical production facilities. Large, concrete hyperbolic towers are used to supply cooling water to electricity production plants operated by the electric utilities. A very large area of application for cooling towers is the area of comfort cooling, or air conditioning systems. In these systems, evaporative cooling equipment is utilized to provided cooling water needed in the condensing operations of the refrigeration system.
Crossflow type evaporative cooling towers could be utilized in either comfort cooling or industrial cooling applications. Crossflow cooling towers typically include a heat transfer surface often comprising a plurality of fill sheets grouped together and supported by the tower structure. Water is distributed from a distribution system gravitationally downwardly through the fill sheets, spreading out across the fill sheets to maximize the water's surface area. As water flows down the fill sheets, air is drawn across, or blown through, the fill sheets in a direction that is 90.degree. transposed from the direction of water flow. As the air contacts the water, heat and mass transfer occur simultaneously, resulting in a portion of the water being evaporated into the air. The energy required to evaporate the water is supplied from the sensible heat of the water which is not evaporated. Accordingly, the temperature of the non-evaporated water remaining in the tower is reduced and cooling is accomplished. The cooled water remaining in the tower is typically collected in a cooled water sump which is generally located at the bottom of the tower structure. From this collection sump, the water is pumped back to the heat source where it picks up additional waste heat to be rejected to the atmosphere. The air into which the water is evaporated is exhausted from the tower.
The design of the water distribution system in a crossflow type cooling tower is important for maximum operating efficiency of the equipment. The purpose of the distribution system is to evenly distribute the hot water to be cooled to the underlying heat transfer surface. Uneven distribution of water to the heat transfer surface will reduce the available air-to-water interfacial surface area which is necessary for heat transfer. Severe maldistribution of the hot water to be cooled may result in air flow being blocked through those areas of the heat transfer media which are flooded with water while at the same time causing air to pass through those areas of media which are starved of water.
Distribution systems used on crossflow cooling towers are generally of the gravity feed type. Such systems typically comprise a basin or pan which is positioned above, and extends across the top of, the heat transfer media. Water nozzles, or orifices, are arranged in a pattern in the bottom of the basin. Distribution systems are typically designed to receive water from above and distribute the water to the nozzles within the basin.
The nozzles operate to pass water contained in the basin through the bottom of the basin and then to break-up the water into droplets and uniformly distribute the water droplets across the top of the heat transfer media. The amount of water which passes through the nozzles depends upon the size and type of the nozzle and the head of water above the nozzle. For ease of design and manufacture, it is desirable for a given basin to contain nozzles of only one size and type. As a result, the major variable affecting the rate of water flow through the various nozzle within the basin is the head of water above the nozzle. Accordingly, it is critical to uniform water distribution that the head of water above the nozzles be equivalent throughout the distribution basin.
Due to the size of the typical crossflow cooling tower, it is often difficult to achieve uniform water head within the distribution basin. Generally, the hot water to be cooled is supplied to the distribution basin from a single pipe centrally located above the basin. In most cases, the basins are 8-12 feet in length. As a result, the water must travel at least 4-6 feet within the basin to reach the nozzles furthest from the supply pipe.
Further complicating the situation is the fact that the water flow rates within a single basin may range from 300 gpm up to 2000 gpm, and more. Flow of this magnitude within a basin of average size creates a substantial degree of turbulence making uniform water head within the basin difficult to achieve. In addition, when water flow rates approach maximum levels, the velocity of water traveling from the center of the basin to the far edges of the basin reach very high levels. Such velocities can cause the water to "shear" across the tops of the nozzles close to the inlet pipe, not allowing the water to turn downward through the nozzles in this area. Such a condition can cause a reduced flow through these nozzles even though sufficient water head exists.
Various methods have been utilized to promote even water distribution in crossflow cooling towers. One such method incorporates the use a diffuser box. The hot water supply piping is connected from above to the diffuser box which is centrally located above the basin. The diffuser box has openings in its bottom which when taken as a whole, have a greater cross-sectional flow area than the hot water supply piping. Accordingly, the velocity of the water exiting the diffuser box is less than the velocity of the water exiting the supply piping. Such boxes also generally contain internal baffles to assist in directing the water out of the bottom of a box at an angle toward the basin edges rather than directing the water vertically downward into the basin.
Another method of providing uniform water distribution to a cooling tower having a basin fed from a centrally located overhead supply piping is described in U.S. Pat. No. 4,579,692. The distribution system described in this patent utilizes a stilling chamber and a flume which is positioned within the distribution pan. The longitudinal axis of the flume is aligned with the longitudinal axis of the basin. One end of the stilling chamber is connected to the hot water supply piping and the other end is connected to the flume at its center, effectively dividing the flume into two sections, each section extending from the center of the basin to one edge. The hot water from the supply piping flows into the stilling chamber and then into the flume. As the water enters the flume, it is divided into two equal streams which flow in opposite directions. As the water is flowing down the flume, it overflows the sides of the flume into the basin thereby providing uniform water distribution throughout the length of the basin.
In other crossflow cooling towers, the hot water to be cooled has been fed to the distribution pan by the use of a flume positioned at the back side of the distribution pan with the longitudinal length of the flume being parallel to the longitudinal axis of the distribution pan. In these cases, the hot water is fed to the center of the flume from above.
In one such arrangement, the flume included a baffle which was sloped downward from the center of the front side of the flume to the ends of the flume. The baffle was positioned above an opening in the bottom of the side of the flume adjacent to the section of the distribution pan in which the nozzles were located. Water would flow down into the flume and a portion would be directed to the ends of the flume by the sloped baffle. The water would be assisted in turning toward the nozzles by two vertical weirs positioned toward the center of the flume, perpendicular to the longitudinal axis of the flume in the distribution pan and extending from underneath the flume into the distribution pan. The water would exit the flume side adjacent to the nozzles and would flow over a sloped weir positioned parallel to the flume and between the flume and the section of the basin containing flow nozzles.
In another such arrangement which has been used for small crossflow towers, the hot water to be cooled would be fed from above the basin to a flume which was positioned above the distribution basin. The water would be deflected toward either side of the flume by a deflecting angle positioned directly underneath the hot water supply piping. The hot water would flow toward the edges of the flume and would flow down into two openings positioned in the back corner and at the bottom of the flume and would then flow underneath the flume and into the distribution pan containing the flow metering nozzles.
Although the methods described have been successfully utilized to provide even water distribution to a distribution basin where the hot water to be cooled is supplied from above the center of the basin, it is advantageous for several reasons if the hot water can be supplied to the basin from underneath. For example, a bottom-fed distribution system would require less pump energy than an top-fed system since the water would not have to be raised to a level above the basin. Also, a cooling tower utilizing a bottom-fed distribution system would require less field labor to install and would be more aesthetically pleasing as it would eliminate unsightly pipework above the cooling tower which must necessarily be present in a top-fed distribution system.
In distribution systems of the bottom-feed type, it is generally impractical to centrally locate the hot water supply piping in the distribution basin due to the presence of the heat transfer surface underneath the basin--though this arrangement would be preferred from a water distribution viewpoint. It is also impractical to locate the hot water supply pipe at the center of the back, inner side of the distribution basin due to the presence of the fan in that area of most crossflow cooling towers. Accordingly, one possible location where fluid may be supplied to a bottom-fed distribution system without unnecessarily increasing the overall size of the cooling tower and while maintaining the tower's pleasing aesthetic appearance is to feed the distribution system asymmetrically from one back corner of the distribution pan.
In a bottom-fed distribution system where the point of supply is at one corner of the distribution pan the distance within the basin from the supply point to the nozzle furthest away is over twice as large as in the centrally located overhead system. Additionally, the volume of water per unit of flow area is also approximately doubled, thereby increasing the possibility of water turbulence within the basin.
One method that has been used to feed a distribution basin from one corner involved laying a perforated pipe inside the basin with the perforated section of the pipe being centrally located in the basin. In effect, the water was piped to the center of the basin and then dispersed through the perforations. This method provided satisfactory distribution at relatively low water flows, however, at high water flows like those associated with a typical crossflow cooling tower, the distribution pipe size required became too large to fit within the basin.